One of the challenges involved in harvesting oil from algae involves finding an efficient technology for rupturing the algae's outer cell membrane in order to extract the oil contained within the algae cell. Considerable efforts have been dedicated to efficient cell lysis, as discussed below, but the technologies which have been developed each have one or more disadvantages including high energy costs, lengthy time requirements, expensive equipment, and low yield.
Harvesting
Known forms of harvesting algal cells fall into three general categories: biomass recovery, dewatering and drying. Specific technologies within each category are discussed below.
Biomass Recovery
Centrifugation uses equipment to spin the algal medium. The centrifugal force of the machine used causes dense material to move away from the spinning axis. This forces nearly all the water to flow outward and leaves a much more concentrated mixture of the algae which can be separated out since it is less dense than water. Centrifugation has several disadvantages. The equipment required for this process is fairly expensive. The equipment also consumes a lot of energy since the centrifuge must run at hundreds, if not thousands, of RPMs for an extended amount of time. Finally, the intense use of energy diminishes the net energy balance upon arrival at the crude biodiesel stage. For purposes of this disclosure Net Energy Balance is equal to the Final Energy Output minus Process-related Energy Inputs.
Decantation is a method of separating mixtures. The process involves pouring a solution from one container into another. If done correctly the algae can be separated from most of the water and any sediment in the solution. The primary disadvantage of decantation is that it involves fairly expensive equipment. While decantation has lower energy requirements than centrifugation, most decanters are used in combination with a centrifuge.
Flocculation involves one of several means for bringing objects suspended in a mixture out of the mixture in the form of flakes or lumps.
Autoflocculation causes the algae to flocculate and form “lumps” by stopping the supply of carbon dioxide to the algae. One disadvantage is that autoflocculation usually requires some extra equipment, but it is fairly inexpensive. However, this routine can take a fairly long time.
Bioflocculation uses organic material to cause the algae to group together and form lumps. For example, chitosan, a linear polysaccharide derived from shellfish, is often used. A secondary process is required to filter the chitosan out of the algae before it reaches the crude biofuel stage. This secondary stage requires additional costs for machinery and energy. In addition, chitosan can be expensive.
Chemical flocculation is a method which separates algae from water by using chemicals. The main disadvantage of chemical flocculation is that the chemicals required are very expensive. In addition, since the chemicals used in the process usually bond to the algae in one form or another, the algae must be separated from the chemical flocculants. Removing these chemicals is expensive and time consuming. Also, many known flocculants like ferric chloride, which is commonly used in the industry, can be very harmful to the environment and are frowned upon by most organizations involved in the creation of biofuels. Ferric chloride is fairly toxic and known to cause harm to the environment.
Froth flotation is another method of separating algae from its growth solution. This process achieves separation by increasing the pH of the solution and bubbling air into the solution along with a basic chemical. Eventually, this causes the algae cells to aggregate in a foam or froth at the surface of the solution. Although the equipment needed is not very expensive, the basic chemicals can be expensive depending on which ones are used. In addition, secondary equipment is needed to treat the post-process solution since it cannot be released into the environment with the elevated pH resulting from the method.
Microfiltration is a process in which the algal solution is pumped through a micro fabricated filter or forced through a membrane. The algae cells gather on one side of the filter or membrane, while the smaller water molecules pass through into a separate tank. This process can be somewhat expensive and its usefulness is limited since it only segregates the algae. The algae still must be lysed so that the oils may be released to form biofuel.
Dewatering
One method of dewatering is to use a draining tank to remove the algae from the solution through a slow cycle of filtering and pumping. This is done by pumping the solution into a tank and then draining the water from the bottom of the tank. This method can be inexpensive, but it is very inefficient.
Once algae has been separated from most of the solution it can be further dewatered using a mechanical press. An apt mechanical press is a lot like a giant vice such as a screw, expeller or piston. The algae is then put under pressure for several hours and often attains very low water content after this process. The primary disadvantages of this method are that it is very time consuming and produces low yields.
Drying
There are numerous techniques for drying out algae. Drum drying is a method used for drying out algae into a film or paste using a large rotating drum that slowly applies heat. The dried algae film or paste is then scraped off the drum surface. Rotary drying is much like drum drying except that an air pump is used to alter the pressure in order to evaporate water. Freeze drying is a dehydration process which works by freezing the subject material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to the gas phase. Solar drying uses glass and lenses to focus and trap heat from the sun. Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. Spray drying is the preferred method of drying of many thermally-sensitive organic materials such as algae.
Most of the drying processes require expensive equipment and use high amounts of energy which has a significant negative impact on the Net Energy Balance. The only exception to this is solar drying which takes an extremely long time and its use is practically limited to very small batches.
Extraction
After the algae have been harvested, oil may be extracted from the algae in the following ways.
Ultrasonic-assisted extraction uses sonochemistry to assist with the extraction of algal oil. Most equipment uses an ultrasonic reactor. The process uses ultrasonic waves to generate bubbles. These bubbles are usually created within a solvent. Once created the bubbles collapse and create small shock waves that break the cell wall. The material inside of the algae cell will spread out into the solvent. The major disadvantage of ultrasonic-assisted extraction is extremely high equipment and maintenance costs. Moreover, the solvents used must be filtered out of the solution resulting in additional costs. The Net Energy Balance is severely impacted by this method.
Mechanical expulsion is the act of extracting oil from algae through a special press or piston operated machine. This process can be very time consuming and has a limited capacity.
Solvent extraction is a process not unlike flocculation, where a solvent is mixed into the algal mass. The chemicals used perforate the cell wall and bind to the triglycerides (oil) within the algae cell. Suitable chemicals for the process include benzene, hexane, and petroleum ether. A serious disadvantage with solvent extraction is that many of the chemicals used can be extremely toxic and care must be taken to avoid exposure to vapors or direct contact with the skin, either of which can result in serious physical problems. Benzene, for example, is classified as a carcinogen. Some of these chemicals are also very expensive.
Supercritical fluid extraction is the process of separating one component (the extractant) from another (the matrix) using a supercritical fluid such as CO2 as the extracting solvent. This method liquefies CO2 using various chemicals and requires extremely high pressure. Heat is then applied until the elements within the solution reach a liquid or gas state. The CO2 is then added, to extract the oil from the algae's cell. Although, this is one of the most efficient means to extract oil from algae, the process needs specialized equipment, most of which is very expensive and energy intensive.
Enzymatic extraction uses enzymes to degrade the cell walls with water acting as the solvent. Through enzymatic extraction, the algal solution is mixed with enzymes to weaken the wall of the cell. In this process water acts as the solvent. The primary disadvantage of enzymatic extraction is that the high cost of producing and then filtering out the enzymes makes this method unrealistic with the present technology.
Sonication, or ultrasonic extraction, is a technique which uses ultra-low and ultra-high sound waves to generate bubbles within the algal solution. These alternating frequencies cause the bubbles to burst which rupture the cell walls, releasing the oil within. The disadvantage of sonication is that equipment is very expensive and needs much more maintenance than other forms of extraction.
Osmotic shock, also known as osmotic stress, is a method that induces a sudden change in the osmotic pressure around the algae cell. Usually achieved through the addition of a salt, the cells membrane weakens and eventually breaks down due to osmotic pressure shifts. By breaking down the membrane, the oil is released from the cell. Very few organizations have achieved a working model of this process, and success has been achieved with a very few unique strains of algae. At this time it is thought to be an unrealistic method of extraction for oil producing algae.
In view of the state of the art, there is a need for a simple, inexpensive, and energy efficient mechanism for harvesting biofuel from algae cells.